1. Field of the Invention
The present invention relates to a ferroelectric memory and a method of testing the same and, more particularly, to a ferroelectric memory which is tested by application of a disturbance voltage in order to test, e.g., the hysteresis characteristic of a ferroelectric cell, and a method of testing the same.
2. Description of the Related Art
The conventional tests of the hysteresis characteristics of ferroelectric cells are proposed in, e.g., U.S. Pat. Nos. 5,661,730, 5,991,189, and 5,822,237.
The test methods disclosed in these patent references, however, do not define that after data is written in a ferroelectric cell, data read should wait until depolarization well advances. Therefore, data may be read out immediately after it is written or before depolarization well occurs, so hysteresis characteristics corresponding to actual uses cannot be tested. It is also impossible to screen a ferroelectric cell having a hysteresis curve with a low coercive voltage.
FIG. 1 of J. Appl. Phys. 75(1), 1 Jan. 1994 shows changes in PZT film with time caused by depolarization. According to the measurement results, after data is written in a ferroelectric cell, read should wait for 10−3 to 100 sec to allow attenuation of the amount of polarization to stop. Accordingly, a test by which data is written and then read out requires a long time if the data is read out after depolarization well advances, and this increases the cost.
The present applicant has proposed a test method capable of reducing the test time in Jpn. Pat. Appln. KOKAI Publication No. 2002-313100. This test sequence is executed following the procedures indicated by steps 1 to 8 below.    (1) Conduct a function test, and calculate the yield    (2) Write initial pattern data    (3) Bake    (4) Calculate the SS yield (same state yield) by reading out the initial pattern data    (5) Write reverse pattern data    (6) Apply a microvoltage equal to or lower than a coercive voltage to a cell capacitor in a direction to weaken polarization    (7) Read out the reverse data pattern, and calculate the OS yield (opposite state yield)    (8) Write the next pattern
In the above test sequence, before the test of reading out the reverse data pattern in step 7, step 6 of applying the microvoltage equal to or lower than the coercive voltage in the direction to weaken the polarization of the cell capacitor is executed. A hysteresis curve immediately after the reverse data pattern is written in step 5 has a sufficiently large amount of residual polarization (or remnant polarization). Since step 6 is executed, however, even a large amount of residual polarization which requires a long time to depolarize attenuates, and this is effectively equivalent to performing depolarization within a short time period. Accordingly, a low read potential equivalent to the amount of polarization after depolarization is output to a bit line.
Note that it is experimentally known that this test does not reduce the amount of polarization of a cell which depolarizes within a short time. That is, even when cells requiring long and short time periods to depolarize are mixed, the addition of step 6 makes it possible to read out, from all cells on the surface of a chip, a bit line potential corresponding to the amount of residual polarization after depolarization within a short time period.
Also, a cell having a low coercive voltage fails by polarization reversal when the microvoltage is applied in step 6. Accordingly, a ferroelectric cell having a hysteresis curve with a low coercive voltage can be screened.
As described above, in the test sequence described in Jpn. Pat. Appln. KOKAI Publication No. 2002-313100, the addition of the process in step 6 makes it possible to give the amount of polarization equal to that after a cell capacitor has depolarized within a short time period, and conduct the test in step 7 for polarized data which has effectively depolarized.
In the technique described in Jpn. Pat. Appln. KOKAI Publication No. 2002-313100, however, a disturbance voltage can be applied to only one cell at one time, and it is necessary to form a circuit which generates a microvoltage for disturbance.
Accordingly, there is still room for improvement in respect of the speed and simplicity of the test.